This invention relates to solventless, internally plasticized epoxy compositions which cure rapidly in ultraviolet light. A suitable photoinitiator, such as triphenylsulfonium hexafluorophosphate [(.phi.)-.sub.3 S.sup.+ PF.sub.6.sup.- ], is required. Sunlight or UV lamps may be used as radiation sources. The mechanism of cure is cationic rather than free radical. When a photon strikes the photoinitiator, a strong acid (HPF.sub.6) is generated. This acid initiates crosslinking through oxirane rings resulting in a network polymer.
A comparison of this invention with related technologies is shown in Table I.
TABLE 1 __________________________________________________________________________ Conventional Conventional Cycloaliphatic Comparison Alkyd Epoxy Epoxy Invention __________________________________________________________________________ Number of Components One Two One (plus One (plus pho- (plus drier) photoinitiator) toinitiator) Cure Type Autoxidation Chemical UV UV Cure Mechanism Free Radical Addition Cationic Cationic Cure Time Moderate to slow Moderate to slow Rapid Rapid Latent Cure Yes No Yes Yes Solvent Use Yes Yes No No Externally Plasticized No No Yes No Internally Plasticized Yes Yes No Yes (by fatty acid) (by curing agent) (by fatty acid) __________________________________________________________________________
Conventional alkyds are oil modified polyesters. Without oil modification, the polyester is a brittle solid. By incorporating unsaturated fatty acids into the polyester, a film with the desired flexibility can be obtained. Alkyds are comprised of an anhydride or diacid, a polyol, and unsaturated fatty acids. Examples of these would be phthalic anhydride, glycerine, and tall oil fatty acid. Vegetable oils, such as soybean oil, contribute both polyol and unsaturated fatty acids to an alkyd formulation. The cure of an alkyd is through autoxidation by a free radical mechanism. The sites of crosslinking are carbons adjacent to unsaturation. Alkyds are one of the most widely used protective coatings due to their durability and relatively low cost. Alkyds typically contain solvents for viscosity reduction.
Conventional epoxy resins are multifunctional glycidyl ethers derived from the reaction of bisphenol A and epichlorhydrin. The oxirane rings of these resins react with amine curing agents, such as reactive polyamides, by chemical addition. The resulting polymer is widely used in protective coatings when superior corrosion and water resistance is required. Solvents are used to reduce the viscosity of the two component reaction mixture.
Cycloaliphatic epoxy resins, such as (3,4-epoxycyclohexyl)methyl-3,4-epoxycyclohexane carboxylate ##STR1## have previously been employed as solventless epoxy compositions which cure rapidly in ultraviolet light. These resins produce rigid films without the addition of a plasticizer. A flow modifier is typically added to prevent surface defects. Reactive diluents, such as propylene glycol monomethylether, may be added to reduce the viscosity of the formulation.
Cationic polymerization of epoxy resin materials by the use of certain radiation sensitive aromatic onium salts of Group VIa elements was disclosed in U.S. Pat. No. 4,058,401 to Crivello. Epoxy resins polymerizable to a higher molecular weight were identified. Examples of photoinitiators identified were triphenylsulfonium hexafluoroarsenate, triphenylsulfonium hexafluoroantimoniate, and triphenylsulfonium tetrafluoroborate. Disclosure of this type of photoinitiator was expanded by Crivello in U.S. Pat. No. 4,161,478 which specifically addresses triphenylsulfonium hexafluorophosphate. A method for effecting the cationic polymerization of epoxy resins with this type of photoinitiator was disclosed by Crivello in U.S. Pat. No. 4,138,255.
Photocopolymerizable compositions based on epoxy and hydroxyl-containing organic materials were disclosed by Smith in U.S. Pat. No. 4,256,828. A process for this polymerization was disclosed by Smith in U.S. Pat. No. 4,318,766. Crivello discussed the use of a hydroxy functional flexibilizer in U.S. Pat. No. 4,175,972. These patents also involved the use of photosensitive aromatic sulfonium salts.
Simultaneous free radical and cationic cure of mixtures of certain oxirane containing and aliphatically unsaturated containing organic resins was addressed by Crivello in U.S. Pat. No. 4,245,029. Arylsulfonium salt-solvent mixtures were disclosed as photoinitiators by Crivello in U.S. Pat. No. 4,273,668. Propylene carbonate was specifically identified as a solvent. Additional photoinitiators were identified by Crivello in U.S. Pat. Nos. 4,407,759 and 4,417,061. General structures for these photoinitiators are ##STR2## respectively.
It is an object of the present invention to produce a durable ultraviolet curable epoxidized alkyd resin coating that cures through a cationic mechanism. It is a further object to produce a durable ultraviolet curable epoxidized alkyd coating that is internally plasticized to produce hard but not brittle films without the use of external plasticizers.
It is a further object to produce a durable ultraviolet curable epoxidized alkyd coating which is water resistant without thermal post-curing.
It is a further object to produce a durable ultraviolet curable epoxidized alkyd coating which is uniform without the use of flow modifiers.
It is an object of the invention to produce a film which is internally plasticized through the use of tall oil fatty acids. These fatty acids act as an internal plasticizer due to their long aliphatic chains. Further, they provide a site for curing by epoxidation of their unsaturation.